Idaho WAS a rival many years ago - PAST tense................Boise State doesn't play them anymore (at least not in football - occasionally in basketball).
B.Y.U. is certainly not a "rival" of Boise State. Those two teams have only played a few times.
T.C.U. could have become a rival if they had stayed in the Mtn. West - but they moved to the Big 12 (with 10 teams).
Nevada and Fresno State are the two principal rivals of Boise State.